Method of suppressing saturation effects in gyromagnetic devices



Aug." 28'; 1962 Filed June 22, 1960 E. M. GYORGY ETAL 3,

METHOD OF SUPPRESSING SATURATION EFFECTS IN GYROMAGNETIC DEVICES 2Sheets-Sheet 1 FIG. I

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as- 1 34 N W W" H' x" I s 3/ GVROMAG/VE 77C MArER/AL 35 2 A TTORNEV 2Sheets-Sheet 2 E. M. GYORGY ETAL E. M GVORGV Z 'H. E. 0. sea V/LjflD/KNEV A'u g. 28, 1962 METHOD OF SUPPRESSING SATURATION EFFECTS INGYROMAGNETIC DEVICES Filed June 22, 1960 United States 3,051,917 METHODOF SUPPRESSING SATURATIGN EFFECTS IN GYROMAGNETIC DEVICES Ernst M.Gyorgy, Morris Plains, and Henry E. D. Scovil,

New Vernon, N.J., assignors to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York Filed June 22,196i Ser. No. 38,938 8 Claims. (Cl. 33 33l) This invention relates toelectromagnetic wave devices using gyromagnetic materials and, inparticular, to means for eliminating the anomalous attenuation etfectsproduced by such gyromagnetic materials at high power levels.

It has been observed that materials of the type having the propertiesdescribed by the mathematical analysis of D. Polder, PhilosophicalMagazine, volume 40, pages 99 through 115 (1949'), have certainanomalous attenuation characteristics which were not predicted byPolders theory. This class of materials, a chief one among them beingferrite, is characterized by certain unpaired electron spins whichrespond to a transmitted microwave signal by precessing gyroscopicallyabout the line of an applied magnetic field. The interaction of theseprecession electrons with the applied microwave signal results incertain magnetic properties which have given these materials the namegyromagnetic. Polders so-called small signal theory predicts anattenuation characteristic as shown by the solid curve -10 in FIG. 1 ofthe drawings. Also shown in FIG. 1 is dashed curve 11 representing whatmay be called the large signal response of gyromagnetic materials. Itwill be observed that the large signal response exhibits certainanomalous characteristics in the regions .of two particular appliedfield values which are not predicted by Polders theory and are notpresent at smaller signal levels. Thus, at the field value H theattenuation for large signals is much greater than for small signal-s,while at another field value H the attenuation for large signals is muchless than that for small signals. This large signal behavior ofgyromagnetic materials has been observed by R. W. Damon, Review ofModern Physics, volume 25, pages 239 through 245, January 1953, and byN. Bloembergen and S. Wang, Physical5 Review, volume 93, pages 72through 83, January 19 4.

The effect of this large signal behavior has been to severely limit theoperating range of electromagnetic wave devices employing gyromagneticmaterials.

Gyromagnetic devices can be broadly divided into two classes: thosebiased below gyromagnetic resonance and which depend for their operationupon the effective permeability of the gyromagnetic element and its lowattenuation, and those biased at resonance and which depend upon theeffective high attenuation of the gyromagnetic element.

Because the so-called large signal effects can, in fact, occur atrelatively low power levels, the performance of both classes of devicesover a substantial range of operating signal levels is adverselyeffected by the anomalous characteristics of gyrornagnetic materials.

It is, therefore, an object of this invention to avoid the so-calledlarge signal behavior of gyromagnetic materials.

The anomalous behavior of gyromagnetic materials at high power levelshas been explained as due to the excitation within the material of aclass of short wavelength spin waves. (This is discussed by H. Suhl inan article entitled The Theory of Ferromagnetic Resonance at High SignalPowers, The Journal of the Physics and Chemistry of Solids, volume 1,pages 209-227, April $51,917 Patented Aug. 28, 1962 1957. At high powerlevels the coupling of energy from the uniform precession to the spinwaves is enhanced, substantially modifying the transmission propertiesof the gyromagnetic material.

It is, accordingly, a more specific object of this invention to inhibitthe transfer of power between the uniform precession and the shortwavelength spin waves.

Investigation has shown that the spin waves propagate within thegyromagnetic material in a preferred direction with respect to theuniform precession, and have a finite build-up time. In accordance withthe invention, means are provided whereby the direction of the uniformprecession is changed at intervals comparable to or less than thebuild-up time of the spin waves. By so modulating the sense of theuniform precession there is insufficient coupling between the uniformprecession and the spin waves to enable their growth and propagationwithin the gyromagnetic material. The suppression of the abovementionedshort wavelength spin waves effectively avoids the so-called anomalouslarge-signal behavior of gyromagnetic materials.

The above-stated and other objects and advantages, the nature of thepresent invention, and its various features, will appear more fully uponconsideration of the various illustrative embodiments now to bedescribed in detail in connection with the accompanying drawings, inwhich:

FIG. 1, given for the purpose of explanation, is a graphical andqualitative representation of the attenuation versus applied magneticfield characteristic of gyromagnetic media showing the small signal andlarge signal responses;

FIGS. 2a and 2b, given for the purpose of explanation, are graphical andqualitative representations of the attenuation versus applied powercharacteristics of gyromagnetic media at the field values H and Hrespectively, shown in FIG. 1;

FIG. 3 is a perspective view of an illustrative embodiment of theinvention in which the modulating field is applied in the directionopposite to the magnetizing field;

FIG. 4, given by way of explanation, is a graph showing the hysteresisloop of a typical sample of gyromagnetic material;

FIG. 5 is a perspective view of a second embodiment of the invention inwhich the modulating field is applied oblique to the biasing field;

FIG. 6 shows a third embodiment of the invention illustrative of amethod of reducing instantaneous variations in the transmissionproperties of microwave devices produced by the modulating field; and

FIG. 7, given by way of explanation, shows the manner in which theinstantaneous phase shift of the device shown in FIG. 6 varies under theinfluence of the modulating field.

Referring more particularly to FIG. 1, there is shown, for the purposeof explanation, a graphical and qualitative representation of theattenuation (a) as a function of the applied magnetic biasing fieldcharacteristic of gyromagnetic materials. The term gyromagnetic materialis employed here in its accepted sense as designating the class ofmagnetic polarizable materials having unpaired spin systems involvingportions of the atoms thereof that are capable of being aligned by anexternal magnetic polarizing field and which exhibit a significantprecessional motion at a frequency within the range contemplated by theinvention under the combined influence of said polarizing field and avarying magnetic field component. This precessional motion ischaracterized as having an angular momentum and a magnetic moment.Typical of such materials are the ferromagnetic materials including thespinels such as magnesium aluminum ferrite, aluminum zinc ferrite andthe garnet-like materials such as yttrium-iron garnet.

Solid curve shows this characteristic for small signal levels below acritical value, to be discussed more fully below, and dashed curve 11shows this characteristic for large signal levels above the criticalvalue The behavior of a gyromagnetic medium for small signals has beenexplained on the-theory that in the presence of an applied magneticfield having an amplitude great enough to saturate the magneticmaterial, the unpaired electron spms in the medium line up parallel toone another and tend to behave gyroscopically as a single unit.Therefore, when the frequency of the applied signal is equal to thenatural precession frequency of the electron spins, a resonant conditionexists under which the electron spins are able to absorb large amountsof energy from the signal. Th s condition, which has been called themain gyromagnetic resonance, is shown at the applied field value H inFIG. 1. At all other field values the attenuation is very low and may beneglected.

The simple uniform precession theory used above, however, does notexplain the shape of the attenuation characteristic at large signallevels, represented by dashed curve 11 in FIG. 1. At these large signallevels, the attenuation at main resonance becomes substantially lowerand the resonance curve becomes substantially broader than at smallsignal levels. Furthermore, a second resonance, which may be termed thesubsidiary resonance, appears at an applied field value of Hsubstantially less than H An attempt will be made below to explain thisanomalous behavior of polarized gyromagnetic media at high signallevels.

The small signal theory states that a microwave signal passing through apolarized gyromagnetic medium is coupled to the electron spins withinthe medium by means of the high frequency magnetic field components ofthe applied signal. The electron spins are thus driven en masse toprecess gyroscopically at some angle about the line of the appliedmagnetic field. Not taken into account by this small signal theory isthe coupling between this uniform precession of the electron spins andcertain small perturbances in the electron spin system which may becalled spin waves.

A gyromagnetic medium is continually in a state of thermal agitation,resulting in a minute and somewhat random misalignment of the electronspins. These perturbances can, by means of a Fourier analysis, beresolved into a series of waves, called spin Waves, which are allcoupled to each other and to the uniform precession by means ofinterspin magnetic forces and electrostatic forces called exchangefields. A relatively narrow band of these spin waves, which may becalled the preferred band, is much more strongly coupled to the uniformprecession than the remainder of the spin waves due to a correspondencebetween their resonances in frequency and direction. The spin wavesystem, and especially the preferred band, can, by means of thiscoupling, absorb energy from the uniform precession. However, underconditions within the scope of the small signal theory, the energy lossto the spin wave system is sufliciently small to be negligible.

The condition of subsidiary resonance, represented by H in FIG. ll, willnow be investigated. When biased below resonance, only very smallamounts of energy can be coupled to the uniform precession due to thelack of correspondence between the applied frequency and the naturalresonant frequency of the uniform precession. However, a small increasein the applied signal will nevertheless raise the excitation of theuniform precession slightly, allowing small amounts of energy to betransferred to the preferred spin wave band and thence to the remainderof the spin wave system. Eventually this energy is transmitted to thecrystal lattice to be dissipated as heat. Since the excitation level ofthe spin wave system has not changed appreciably, the attenuationoffered to far the conditions have remained within the scope of thesmall signal theory.

As the power level of the applied signal continues to increase, however,a critical point is reached where the preferred band of spin waves canno longer transfer energy to the remainder of the spin Wave system asfast as it is being received from the uniform precession. At this point,called the critical power level, the preferred band goes to a higherstate of excitation to accommodate the increase in energy level. Theexcitation of the preferred band tends to build up rapidly since thecoupling thereto is nonlinear, increasing with increasing signal level.This band, being resonant with the uniform precession, is now morestrongly coupled to the uniform precession and therefore receives evenmore energy from the uniform precession. This further increases theexcitation level of the preferred band, allowing even further amounts ofenergy to be coupled thereto. This build-up cycle continues until thepower absorbed by the preferred band is just sufiicient to balance thelosses of the resonant system. It can be seen that an unstable conditionexists at the critical power level which results in large amounts ofenergy being absorbed from the applied signal. This results in a largeincrease in the attenuation offered to the applied signal. Any furtherincrease in the power level of the applied signal is substantially alldiverted into the preferred spin wave band. This condition is shown asthe subsidiary resonance hump in dashed curve 11 of FIG. 1 at appliedfield value H The change in attenuation can more readily be seen in FIG.2a.

In FIG. 2a there is shown, for the purpose of explanation, a graphicaland qualitative representation of the attenuation versus power inputcharacteristic of a gyromagnetic medium biased to a field value H asshown in FIG. 1. It can be seen that the attenuation is very low forpower inputs below the critical power P At this point, however, theattenuation suddenly jumps to a very high value due to the resonancebetween the uniform precession and the preferred spin wave band. Beyondthis point the attenuation decreases slightly but retains substantiallyits high value. The power level at which the run-away condition occursis a function of the magnetic state of the gyromagnetic medium and therelaxation time of the preferred spin Waves.

In the case of the main resonance at an applied magnetic field of H theuniform precession is again cou pled to a preferred band of spin Waveshaving a frequency and direction of resonance closely resembling that ofthe uniform precession. Under this condition, however, the uniformprecession is already absorbing large amounts of energy from the appliedsignal and is therefore near its maximum state of excitation. When thecritical power level is reached and the preferred spin wave band can nolonger get rid of energy as fast as it receives it, the preferred spinwaves go to a higher state of excitation at the expense of the uniformprecession. The removal of energy from the uniform precession decreasesthe coupling of this precession to the applied signal and hence theattenuation olfered to the signal also decreases. Further increases inthe power level of the applied signal result in further excitation ofthe preferred spin waves and a larger decoupling of the uniformprecession from the applied signal. The attenuation therefore decreasesand eventually goes to zero when the uniform precession is completelydecoupled from the applied signal. This condition is shown as thedecline and broadening of the main resonance peak in dashed curve 11 ofFIG. 1 at applied field value H The change in attenuation can morereadily be seen by considering FIG. 2b.

In FIG. 2b there is shown, for the purpose of explana tion, a graphicaland qualitative representation of the attenuation versus power inputcharacteristic of a gyromagnetic medium biased by a field H as shown inFIG. 1. It can be seen that the attenuation is very high for powerinputs below the critical power P At this point, however, theattenuation suddenly drops to a low value. Thereafter, the attenuationcontinues to decrease, approaching zero. The critical power level atwhich the decline in attenuation begins has been found to be governed bythe same factors as govern the critical power level at subsidiaryresonance.

In FIG. 3 a reciprocal phase shifter is shown, modified in accordancewith the invention to eliminate subsidiary resonance effects and tothereby produce low-loss phase shift at radio frequency power levelssubstantially greater than the critical power level for the gyromagneticmaterial. Specifically, the phase shifter comprises a guide 30 ofbounded electrical transmission line for guiding wave energy, which maybe a rectangular waveguide of the metallic shield type having a .wideinternal cross-sectional dimension of at least one-half wavelength ofthe wave energy to be conducted thereby and a narrow dimensionsubstantially one-half of the wide dimension.

Included within guide 30 are means for imparting a phase delay to thewave energy propagating therethrough. In particular, disposed withinguide 30 is a thin vane 31 of gyromagnetic material. Vane 31 issymmetrically disposed within guide 30 along the longitudinal guide axisequally spaced from both narrow walls, with the long dimension of vane31 extending longitudinally along the guide, parallel to the guidewalls.

Vane 31 is biased by a steady magnetic field at right angles to thedirection of propagation of the wave energy in guide 30. As illustratedin FIG. 3, this field may be supplied by an electrical solenoid having amagnetic core 32 and pole pieces N and S bearing upon. the wide walls ofguide 30- in a region substantially coextensive with the gyromagneticvane 31. Turns of wire 33 are wound about core 32 and connected througha potentiometer 34 to a source of magnetizing current 35.

The operation of the phase shifter shown in FIG. 3 is based upon theeffective permeability presented to the propagating wave. Since resonantabsorption represents a loss for these applications, these devicesoperate in a range of applied magnetic fields between zero and thatrequired to initiate the resonant phenomenon. In particular, the regionof magnetic saturation is of primary importance since the effectivepermeability is greatest in this region. At power levels below thecritical power level, low-loss phase shift is readily obtained. However,above the critical power level coupling between the imiform magneticprecession and the spin waves gives rise to the above-describedsubsidiary resonance effect which, for all practical purposes,substantially destroys the usefulness of the phase shifter.

In accordance with the invention the tendency to couple energy to thespin waves is inhibited by changing the direction of magnetizationwithin the gyromagnetic vane 31. In the embodiment of the inventionshown in FIG. 3 this is done by modulating the steady biasing field bymeans of a high frequency signal having an amplitude and frequency whichwill be explained in greater detail hereinafter. The modulating field isimpressed upon the magnetic core 32 by turns of wire 37 which connect toa high frequency energy source 36.

For simplicity, source 36 is shown in FIG. 3 as a separate generator. Itis understood, however, that source 36 would generally be associatedwith a power level detector that would monitor the power level in guide311 and only gate source 36 on when the power level in guide 30 exceededthe critical power level of the gyromagnetic medium.

In operation, potentiometer 34 is adjusted to produce a steady biasingfield having an amplitude sufliciently large to produce saturation invane 31. So biased, the magnetization throughout the material is alignedparallel to the direction of the biasing field. Wave energy, having anamplitude less than the critic-a1 amplitude for the gyromagneticmaterial, will propagate along guide 3%? and past vane 31 withsubstantially little or no attenuation. Under this condition source 36is gated off. As the power level of the propagating wave increases andapproaches the critical power level, source 36 is gated on. The outputof source 36 is a wave having an amplitude and frequency to reverse thedirection of magnetization at a rate related to the spin wave build-uptime in vane 31.

To determine the amplitude of the modulating field necessary to reversethe magnetization when the magnetic material is biased at or abovesaturation, reference is made to FIG. 4 which shows a typical hysteresisloop for the gyromagnetic material. Specifically, FIG. 4 shows therelationship between the magnetomotive force or magnetizing field H andthe magnetic flux density B. Assuming the biasing magnetization to be -Hthe magnetic state of the material is that given by point (1) on FIG. 4.To reduce the magnetic flux, B, to zero from point (1) would require areverse magnetomotive force of H -l-H where H is the coercive force forthe material. This is indicated at point (2). To now reverse themagnetic flux in the magnetic material to some point (3), theapplication of an additional magnetomotive force AH is necessary.

It should be noted, however, that to go from state (1) to state (3)requires a finite time. The mere application of a reverse magnetomotiveforce will not, instantaneously, reverse the flux within thegyromagnetic material. Since it is necessary for the purposes of theinvention to change the magnetization in a time that is short comparedto the spin wave build-up time, the amplitude of the reversing forcemust be adjusted accordingly. Specifically, if the spin wave build-uptime is T the switching time 'r should be greater than T Themagnetomotive force H required to switch at this rate is then where Sthe switching coefiicient, and H the threshold field, are constant ofthe material and are determined experimentally. A typical value of S is0.2 oe.;isec., while H is approximately equal to 2H The spin wavebuild-up time, T being a function of the material, its geometry and theradio frequency power level, is also determined experimentally. This canbe done by suddenly applying a radio frequency wave greater than thecritical power level to the gyromagnetic element biased below resonance.Momentarily the output will rise to full transmission. As power iscoupled to the spin waves and the spin waves build up, the output willexponentially fall off until a lower steady state output is reached. Thetime for the output to decline to approximately 37 percent of the peakoutput is one time constant, or 'r In a preferred embodiment is madeequal to T 10.

In the embodiment of the invention shown in FIG. 3, given for thepurpose of illustration, it is assumed that the gyromagnetic material istransversely biased to saturation and that the function of the microwavedevice is to introduce phase shift. It is to be understood, however,that for the purposes of this invention the function of the device couldjust as well be to introduce attenuation into the microwave system andfor that purpose the gyromagnetic material is biased to gyromagneticresonance. As was explained hereinbefore, when a resonantly biasedattenuation is operated above the critical power level, the overallattenuation tends to decrease. By modulating the steady biasing field,as explained hereinbefore, coupling between the uniform precession andthe spin Waves is impeded and the tendency for the attenuation todecrease is avoided.

It will also be noted that in the illustrative embodiment of FIG. 3 themodulating field completely reverses the direction of the biasing field.However, as was pointed out, it is only necessary to change the direc- 7tion of the magnetization within the gyromagnetic material. This wouldalso include a change in direction less than 180 degrees. A modificationof the embodi ment of FIG. 3 wherein changes in the direction ofmagnetization less than 180 degrees are utilized as shown in FIG. 5.

The device shown in FIG. 5 comprises a section of waveguide 50, and avane of gyromagnetic material 51 disposed therein. Vane 51 is biased bya steady magnetic field H at right angles to the direction ofpropagation of the wave energy in guide 50. This field may be suppliedby an electric solenoid, by a permanent magnetic structure, or vane 51may be permanently magnetized if desired.

In the embodiment of FIG. 5 the steady biasing field H is modulated bymeans of locally generated magnetic fields which tend to alter thedirection of the biasing field. These local fields are produced by meansof a conductive member 52 which is threaded through the gyromagneticvane 51. As shown in FIG. 5, conductor 52 lies in a plane perpendicularto the electric field in guide 50 and passes through the broad surfaceof vane 51 over a region coextensive with the longitudinal dimension ofthe vane. Conductor 52 is energized by means of the high frequencyenergy source 53.

As before, when the amplitude of the wave energy is less than thecritical level, source 53 is off. As the power level of the propagatingwave increases and approaches the critical level, source 53 is gated on,energizing conductor 52 and producing local magnetic fields aboutconductor 52 in the region of the gyromagnetic element. Specifically,the magnetic field produced by the modulating source 5-3 comprisesclosed loops 54 surrounding conductor 52. The effect of these fieldcomponents is to alter the direction of the net magnetic field over mostof the volume of the gyromagnetic vane, thereby minimizing the tendencyfor energy to couple between the uniform precession and the spin waves.The

amplitude of the modulating field will depend upon the application; thatis, if the device shown in FIG. 5 is a phase shifter, the amplitude ofthe modulating field is adjusted so as to maintain the attenuationthrough the device below a specified maximum for the given operatinglevel. If, on the other hand, the device in FIG. 5 is intended to be aresonant attenuator, then the amplitude of the biasing field is adjustedso as to maintain the attenuation above a specified minimum at thedesired operating level. As before, however, the modulating rate isrelated to the spin wave build-up time for the given gyromagneticelement.

It is apparent from the above discussion that the net efiectivemagnetization within the gyromagnetic element is varied as a function oftime. While the desired effect of this variation is to disrupt thecoupling between the magnetization and the spin waves, it also tends tomodulate the instantaneous phase shift or attenuation produced by themicrowave device. If the resulting overall phase shifter the resultingoverall attenuation is still suflicient for the purpose intended, thismodulation, or ripple, produced by the modulating wave may be tolerable.If, however, the variations produced 'by the modulating wave are notpermissible, corrective measures can be taken. Perhaps the simplestcorrective measure consists in cascading a number of gyromagneticelements and suitably phasing the modulating field impressed upon themso as to reduce the net modulating ripple to a specified minimum level.A simple embodiment of such an arrangement is shown in the structure ofFIG. 6, which is basically a phase shifter of the type described by F.Reggia and E6. Spencer in an article entitled A New Technique in FerritePhase Shifting for Beam Scanning of Microwave Antennas, November 1957,Proceedings of the I.R.E., pages 1514-4517, modified in accordance withthe principles of the invention.

The tic-called Reggia-Spencer phase shifter comprises a pencil ofgyromagnetic material disposed along the longitudinal axis of arectangular section of waveguide. In this type of phase shifter thegyrornagnetic element is longitudinally biased below saturation. Whilemodula tion of the longitudinal magnetic field in accordance with theprinciples of the invention will extend the power handling capabilitiesof this type of phase shifter, the instantaneous phase shift produced bythe device will also vary, thus introducing'what could be anobjectionable phase shift ripple in the output wave.

This difiiculty, however, may be readily obviated by modifying the phaseshifter as shown in FIG. 6. Specifically, the overall phase shift isobtained in two parts by dividing the gyromagnetic element into twoportions and separately controlling the magnetic fields applied to eachof the two portions. The phase shifter shown in FIG. 6 comprises asection of rectangular waveguide 60 within which there are suitablysupported two cylindrical rods 61 and 62 of gyromagnetic material. Rods61 and 62 are longitudinally disposed within guide 60 along the guideaxis and are longitudinally biased by means of solenoids 63 and 64,respectively, mounted outside of waveguide 60. Solenoid 63 is connectedthrough potentiometer 65 to a source of magnetizing current 66.Similarly, solenoid 64 is connected through potentiometer 67 to saidsource of magnetizing current 66.

Since the rods are biased below saturation, the direction ofmagnetization is not parallel to the biasing field but instead variesthroughout the volume of the rods. Thus, the instantaneous direction ofmagnetization can be varied by merely varying the intensity of thebiasing field. Accordingly, the modulating field is applied parallel tothe biasing field by means of the two additional solenoids 68 and 69,each of which extends over a region of guide 60 substantiallycoextensive with one of the rods. Solenoids 68 and 69 are energized fromthe same high frequency energy source 70. However, inserted in thecircuit associated with solenoid 69 is the phase shifter 71 for intro:

ducing a 180 degree phase difference between the modulating current insolenoid 69 and the modulating current in solenoid 68, as will beexplained in greater detail hereinafter.

Curve of FIG. 7 shows the phase shift produced by each of thegyromagnetic rods, in the embodiment shown in FIG. 6, as a function ofthe instantaneous magnetizing field H. Assuming a total desired phaseshift of 18 degrees, the magnetizing field produced by solenoid 63 isadjusted to H producing a phase shift ,8 along rod 61, and themagnetizing field produced by solenoid 64 is adjusted to H producing aphase shift ,8 along rod 62, Where Bl+B2 B3- As the amplitude of thewave energy propagating along guide 60 approaches the critical level,source 70 is energized subjecting rods 61 and 62 to a varying magneticfield component which modulates the phase shift produced by each of saidrods. Under the influence of solenoid 68, the total magnetizing fieldwithin rod 61 will start to increase, causing the total phase shiftproduced by rod 61 to increase in accordance with the variation definedby curve 80. Let us assume that the total magnetizing field for rod 61increases to a point H The total phase shift produced by rod 61 is thenincreased to [3 Because of the degree phase shift produced by phaseshifter 71, the effect of the modulating field produced by solenoid 69is to reduce the total magnetizing field in rod 62 from H to H causingthe total phase shift in this section of the device to decrease from ,8to ,6' Because curve 80 is substantially linear in the region underconsideration, B +;8 is substantially equal to ti -H3 Thus, the totalphase shift through the two sections of the phase shifter remainssubstantially constant even though the individual phase shift in eachsection may vary instantaneously due to the effect of the modulatingfield. It is obvious that by suitably arranging the phasing of themodulating fields, the num- Let us consider rod 61 first.

her of gyromagnetic rods may be increased and the total phase shiftdivided among these additional rods, further reducing any ripple in theoverall phase shift.

In all cases it is understood that the above-described arrangements areillustrative of a small number of the many possible specific embodimentswhich can represent applications of the principles of the invention.Numerous and varied other arrangements can readily be devised inaccordance with these principles by those skilled in the art withoutdeparting from the spirit and scope of the invention.

What is claimed is:

1. An electromagnetic wave transmission device comprising a section ofguided wave path having an element of ferromagnetic material disposedtherein, said material characterized as having a first transmissionconstant for applied signals below a critical power level and a secondtransmission constant different than said first constant for appliedsignals above said critical power level, said material furthercharacterized as having a given spin wave build-up time, means forestablishing a given state of magnetization within said material, meansfor applying electromagnetic wave energy to said section of wave pathhaving a power level greater than said critical power level, and meansfor preventing said material from assuming said second transmissionconstant including means for modulating said given state ofmagnetization at a rate not less than the reciprocal of the spin wavebuild-up time of said material.

2. The combination according to claim 1 wherein the direction ofmagnetization within said material is reversed by said modulating means.

3. The combination according to claim 1 wherein the direction ofmagnetization within said material is caused to change by saidmodulating means by an amount less than 180 degrees.

4. A high power, low-loss, microwave device including an electromagneticwave transmission path having a power saturable ferromagnetic mediumdisposed therein, said medium characterized as having a low positiveattenuation constant for applied signals below a critical power level,but capable of exhibiting a high positive attenuation constant tosignals above said critical power level, said medium furthercharacterized as having a given spin Wave build-up time, means forapplying a steady magnetic biasing field to said medium, means forapplying electromagnetic wave energy to said path having a power levelgreater than said critical power level, and means for preventing saidmedium from exhibiting said high attenuation constant including meansfor modulating said magnetic field at a rate not less than thereciprocal of the spin wave build-up time of said medium.

5. A microwave phase shifter comprising a guided electromagnetic wavetransmission path having an element of ferromagnetic material disposedtherein, said medium characterized as having a low positive attenuationfor applied signals below a critical power level but capable ofexhibiting a high positive attenuation to signals above said criticalpower level, said medium further characterized as having a given spinwave build-up time 'r means for applying a steady magnetic biasing fieldH to said medium in a given direction, means for applyingelectromagnetic wave energy to said wave path having a power levelgreater than said critical power level, and means 1% for reversing thedirection of said biasing field at a rate l/T greater than thereciprocal of the spin wave build-up time for said material, saidreversing field having an amplitude where H is the threshold field forsaid material, and S is the switching coefficient.

6. The combination according to claim 5 wherein the switching rate l/ isapproximately equal to 10/T 7. A phase shifter for electromagnetic waveenerg comprising a section of conductively bounded waveguide supportiveof said wave energy, first and second elongated elements offerromagnetic material disposed in longitudinal succession within saidwaveguide, each of said elements presenting a first propagation constantto wave energy below a given power level and a second propagationconstant to wave energy above said given power level, each of saidelements also having a given spin wave buildup time, means forlongitudinally magnetizing said first element at a first fieldintensity, means for longitudinally magnetizing said second element at asecond field intensity greater than said first intensity, where saidfirst and second intensities are less than that necessary to producesaturation in said elements, means for applying electromagnetic waveenergy to said waveguide having a power level greater than said givenpower level, and means for increasing said first field intensity anincremental amount AH, and means for decreasing said second fieldintensity an incremental amount substantially equal to AH at a rategreater than the reciprocal of said given spin wave build-up time forsaid elements with the variation in said first element being 180 degreesout of time phase with respect to the variation in said second element.

8. A device for electromagnetic wave energy comprising a section ofconductively bounded waveguide supportive of said wave energy, aplurality of n elements of ferromagnetic material disposed inlongitudinal succession within said Waveguide, each of said elementspresenting a first propagation constant to wave energy below a givenpower level and a second propagation constant to wave energy above saidgiven power level, each of said elements also having a given spin wavebuild-up time,

. means for magnetically biasing each of said elements at ReferencesCited in the file of this patent UNITED STATES PATENTS 2,798,205 HoganJuly 2, 1957 2,820,200 Du Pre Jan. 14, 1958 2,847,647 Zaleski Aug. 12,1958 OTHER REFERENCES Wheeler: IRE Transactions on Microwave Theory andTechniques, January 1958, pages 38-39.

